This invention relates to coated optical fibers and to a method of coating optical fibers.
Optical fibers are conventionally coated with a protective coating composition immediately after their drawing. This coating is necessary not only for preventing the fiber from being scratched during subsequent steps but also for protecting the fiber from deteriorative environment and microbending that produces loss of signal transmission.
Since it is difficult for a single layered coating to comply with all requisite properties, it is conventional and commercial practice to provide a multi-layered structure composed of layers having different roles. For example, the primary coating layer directly surrounding the cladding of the fiber is required to have lower Young's modulus, higher elongation and higher refractive index, while the secondary or outer layer is required to have higher toughness and higher abrasion resistance.
In order to meet these requirements, numerous attempts have been made to improve the material employed as the coating film but these all have proven unsatisfactory.
Besides, as the fiber-drawing speed has become more rapid in recent years, needs exist for such a coating composition having a shorter curing time. To this end, radiation, curable compositions are attractive. Usually these compositions, as opposed from thermosetting compositions, do not contain any volatile solvent so as to obviate its evaporation or film setting by heat. It is, therefore, difficult to control the workability of these solvent-free compositions.
We have found that various performance characteristics of coating films applied on optical fibers may be significantly improved by incorporating polymer microparticles to the compositions used for the stated purposes. The polymer microparticles may also improve the workability of the radiation curable, solvent-free compositions.